1. Field of the Invention
Exemplary embodiments of the present invention relate generally to the fields for delivery of gas mixtures to humans inside a magnetic resonance imaging (MRI) scanner. More particularly, exemplary embodiments concern systems and methods for delivery of gas mixtures to humans inside an MRI scanner while monitoring and recording physiological parameters.
2. Description of Related Art
Existing MRI scanners and associated systems do not readily provide for the administration of different combinations of breathing gasses to a human subject located inside the MRI scanner without moving the subject. The ability to administer different combinations of breathing gasses to a subject while conducting MRI scans without moving the subject can provide the ability to accurately measure many important physiological conditions, including for example, cerebrovascular reactivity (CVR), cerebral blood volume, and bolus transit time inside the blood stream.
CVR refers to the ability of blood vessels to dilate upon stimulation and is an important marker of brain's vascular function [1]. Cerebral blood volume (CBV) refers to the amount of blood in the brain. Bolus transit time refers to the time it takes for the gas bolus to travel from one location to another inside the brain's vascular network. There has been an increased interest in quantitative mapping of these physiological properties using MRI in combination with gas challenge. However, the application of this method is limited by the availability of MRI-compatible gas delivery systems.
Taking CVR, for example, the most commonly used method of CVR mapping is maneuvering the concentration of CO2, a potent vasodilator, in the blood by hypercapnia inhalation while monitoring vascular responses using MRI. However, delivering CO2 gas mixture to the subject inside the MRI scanner is not a trivial endeavor. Special considerations are required in designing MRI-compatible gas delivering systems. These special considerations include: (1) all components must be non-metallic, since metal cannot be used inside an MRI scanner; (2) the system should work within a small space that the MRI system and its head coil allow; (3) the system should work with a lying-down position (as MRI scanner requires) instead of sitting up, while keeping the subject comfortable; (4) the physiological parameters, such as end-tidal CO2 (EtCO2) and end-tidal O2 (EtO2), should be recorded accurately with seconds of timing accuracy and stored on a computer for any future use.
In view of these technical challenges, there are limited MRI-compatible gas delivery systems that have been reported to provide CO2 maneuvering in MRI environment [17,18], each of which are complicated and expensive. In addition, these systems require extensive training of the operator and preparation time. The use of a face mask in such systems also dampens the accuracy of EtCO2 and EtO2 recordings as the inspired and expired air is mixed in the mask space where the sampling line is located. These issues largely limit CVR mapping from being a widely available tool in clinical practice.
Compared to baseline vascular parameters, such as baseline cerebral blood flow (CBF)—which can be influenced by factors unrelated to vascular function such as neural activity and metabolic demand—CVR is more specific in reflecting vascular health [2]. During the past few years, CVR measured with MRI has found to be attenuated in many brain disorders such as small vessel diseases [3], arteriovenous malformation [4], Moyamoya disease [5,6], arterial stenosis [7], drug-addictive conditions [8], and normal aging [9]. It has also been shown that CVR can be used to normalize functional MRI (fMRI) signal [10-14] and in the evaluation of brain metabolism [15,16].